Many medical devices contain components made from a plastic material which initially may be white, colored or clear and transparent. Furthermore, medical devices are packaged in containers or envelopes which frequently are made of clear and transparent plastic material. If the device is to be used in a surgical procedure, particularly an invasive procedure, it must be sterilized before or after packaging and usually the whole pack comprising the container and device is sterilized.
The container of the pack is frequently made from a white or clear plastic material, or a combination of white and clear plastic and typical plastic materials are polyurethanes, polyvinylchloride (PVC), polyethylene, polyethylene terephthalate glycolate (PETG), polycarbonate and acrylic polymers, particularly polymethyl methacrylate (PMMA). A medical device of particular interest is an invasive sensor for the determination of analytes in blood and a typical component of such a sensor is transparent acrylic (e.g., PMMA) optical fiber.
U.S. Pat. No. 4,889,407 discloses an optical waveguide sensor in which a preferred waveguide is an optical fiber made from clear PMMA. Such a sensor may incorporate a device for returning light such as that disclosed in U.S. Pat. No. 5,257,338.
Sensors of the type disclosed in U.S. Pat. Nos. 4,889,407 and 5,257,338 are used in a multi-parameter sensor available under the Registered Trade Mark "Paratrend 7" from Biomedical Sensors Limited, High Wycombe, England.
A preferred package for the Paratrend 7 is disclosed in U.S. Pat. No. 5,246,109, which package comprises a number of components made from a plastic material, including a transparent blister pack made from PETG.
A number of sterilization techniques are known in the art and have been used to sterilize plastic-containing packs for surgical or medical procedures, for example, PVC blood bags and various tubing sets, as well as the sensor devices mentioned above. A common sterilization technique utilizes ethylene oxide as the sterilization medium. A disadvantage of this technique is that it takes some time, usually up to two weeks, before the amount of residual ethylene oxide drops to an acceptable level.
An alternative sterilization technique is irradiation by gamma radiation. This has the advantage that there is no contamination by the sterilizing agent. However, a disadvantage of the gamma irradiation technique is that many plastic materials, particularly clear, transparent plastic materials, become discolored by the gamma radiation.
Examples of such discoloration are that PVC turns a greeny brown, polycarbonate turns green, polyethylene yellows and acrylics turn an orangey brown.
Although, in many cases, the discoloration fades somewhat over time if the pack is maintained in the dark, the plastic material never recovers to its original color or clarity. Apart from being aesthetically undesirable, the discoloration may interfere with or reduce the efficiency of the article, particularly when the device is an optical fiber in which discoloration may affect the sensitivity of optical signals, or if the device is one in which clarity is necessary to monitor the state of material within the device, for example, a transparent Y-connector in a tube arrangement where, for example, bubbles in the liquid would not be visible if the transparent wall is badly discolored.
Accordingly, it is highly desirable that discoloration of plastic material in a medical device be reduced and, if possible, the material be restored to its original color to facilitate not only the appearance of the plastic material, but also the effectiveness of the device.
A study of the effect of sterilization, particularly using ethylene oxide or gamma radiation, on rigid thermoplastic resins was reported in an article by Marianne F. Sturdevant entitled "How Sterilization Changes Long-term Resin Properties", Plastics Engineering, January 1991, pages 27-32. The article is primarily concerned with the long-term effects, up to one year, of gamma radiation sterilization on the physical properties of plastic materials, especially styrenic polymers, such as styrene-acrylonitrile copolymer (SAN), general purpose polystyrene (GPPS), high-impact polystyrene (HIPS), and acrylonitrile-butadiene-styrene (ABS); polycarbonate (PC); linear low-density polyethylene (LLDPE) and rigid thermoplastic polyurethanes (RTPU).
The article discusses the effects of gamma radition on the optical properties of the plastic materials and mentions that all materials that were exposed to gamma radiation were discolored to varying degrees and that discoloration increased with increasing dosages. The article also stated that the initial discoloration diminished with time and that for PC and some of the styrene-based polymers exposure of the irradiated sample to mild UV light can accelerate the decrease in discoloration by a phenomenon known as photo-bleaching. However, since the investigation reported in the article was concerned primarily with changes in long term physical properties, such as tensile strength and impact resistance, and the photo-bleaching effect was "solely an optical phenomenon", the reduction of discoloration was not seriously pursued and it was not recognized that the original clarity and lustre of clear and transparent plastic material could be completely restored by controlled irradiation with "blue light" as described hereinafter.
Surprisingly, it has now been found that discoloration induced by gamma radiation sterilization may be substantially diminished and the original color and clarity of the material be substantially restored by exposing the discolored material to electromagnetic radiation having a wavelength within the range of 200 to 600 nm, particularly blue light having a maximum wavelength of 420 nm, for a period of time of less than twenty four hours.
A sterilized pack, such as that disclosed in U.S. Pat. No. 5,246,109, in which discoloration produced by gamma radiation sterilization is removed by the technique disclosed herein, is also within the scope of the present invention.
Electromagnetic radiation having a wavelength within the stated range of 200 to 600 nm is partly within the ultraviolet region of the electromagnetic spectrum, i.e., up to about 390 nm is ultraviolet radiation, and partly within the visible light portion of the spectrum, i.e., from about 390 to 600 nm. The preferred radiation having a peak wavelength of about 420 nm is in the violet or far-blue portion of the spectrum and is herein designated as "blue light". For convenience, and also to clearly distinguish from the gamma radiation mentioned herein, the radiation used in the method of the present invention, both in the ultraviolet and visible light regions of the electromagnetic radiation spectrum, is referred to as "light" having a wavelength within the range of 200 to 600 nm.